1. Field of the Invention
The present invention relates to a ball bearing, and more particularly, to a full ball type deep groove ball bearing which is suitably used as a touchdown bearing in a turbo-molecular pump.
2. Related Art
In general, ball filling slots for mounting balls are formed in full ball type deep groove ball bearings as shown in Japanese patent publication No. JP 11-325085A. The ball filling slot extends over inner and outer rings so as to be formed substantially in a circular-shape. The ball filling slot is designed to have a size slightly smaller than the size of the balls such that the ball smoothly rotates and does not escape therefrom. Namely, the ball filling slot is designed to have a size so as to form a small interference to the balls.
In such a full ball type deep groove ball bearing, for mounting the balls, either of the following methods is used. One is a method of directly pressing a ball placed in the vicinity of the ball filling slot into the bearing. The other is a method of inserting a jig (a wedge) into the ball filling slot or a space between the inner ring and the outer ring in the vicinity of the ball filling slot to widen a gap between the inner and outer rings first, and then inserting the balls into the bearing in a state that the interference is hardly remained.
However, in the method of pressing the ball placed in the vicinity of the ball filling slot into the bearing, if the rigidity of the bearing rings is high, a large force for pressing the ball is required so that the balls and the periphery of the ball filling slot of the bearing ring can be easily damaged. The periphery of the ball filling slot can be easily deformed, when a ball is pressed into the slot. On the other hand, as for the method of inserting the wedge into the ball filling slot or the space between the inner ring and the outer ring in the vicinity of the ball filling slot first and then inserting the balls in a state that the interference is hardly remained, this method cannot be used if the gap between the outer diameter of the outer circumferential surface of the inner ring and the inner diameter of the inner circumferential surface of the outer ring is very small, and the wedge cannot be inserted into the space between the inner and outer rings.
On the other hand, conventionally, there is a turbo-molecular pump in which a rotary shaft is magnetically supported with respect to a housing a magnetic bearing with a non-contact state at the time of normal operation and a ball bearing is used as a touchdown bearing to mechanically support the rotary shaft by contacting with the rotary shaft when a vacuum environment is destroyed due to an operation error (artificial error) of a vacuum apparatus decompressed by the turbo-molecular pump such as a switching error of a valve of the vacuum apparatus for example, or due to an emergency state such as the stoppage of power supply. (See, for example, JP 11-190348A.)
Among turbo-molecular pumps having such a construction, there is a turbo-molecular in which a strong 2-pole permanent magnet (functioning as a rotor of a motor) is fixed at one of the rotary shaft and the housing in order to rotate the rotary shaft in high speed with respect to the housing. In this turbo-molecular pump, overcurrent according to the law of electromagnetic induction occurs in the other of the rotary shaft and the housing in which the 2-pole permanent magnet is not mounted due to a magnetic field generated by the 2-pole permanent magnet, and heat is generated in the other of the rotary shaft and the housing in which the 2-pole permanent magnet is not mounted.
It has been known that the heat is not radiated well to the outside of the pump and remains in the pump when the 2-pole permanent magnet is fixed at the housing so that the heat is generated in the rotary shaft. Therefore, this configuration is not suitable. Accordingly, in the turbo-molecular pump, the 2-pole permanent magnet is usually fixed at the rotary shaft so that the heat due to the mount of the 2-pole permanent magnet efficiently radiates to the outside of the pump.
For manufacturing such a turbo-molecular pump, the manufacturing is generally conducted by mounting a ball bearing in a housing first and then passing a 2-pole permanent magnet fixed to a rotary shaft through an inner circumferential surface of an inner ring. Accordingly, the assembling of the ball bearing and the rotary shaft is simplified.
Under such circumferences, the following problem has been found. If bearing rings of the ball bearing are formed of a metal material having high hardness such as a stainless steel or bearing steel to which a hardening treatment is performed in order to increase an impact strength, the inner ring is magnetized to become a magnet when the 2-pole permanent magnet fixed to the rotary shaft passes the inside of the inner circumferential surface of the inner ring. Accordingly, overcurrent may flow in the rotary shaft by a magnetic field generated by the magnetized inner ring, so that the heat is generated in the rotary shaft. In other words, upon normal operation, when the rotary shaft rotates relative to the housing and the ball bearing as the touchdown bearing is in such a condition that the outer ring is fixed to the housing and that the inner ring is not brought into contact with the rotary shaft and does not rotate relative to the outer ring, the heat is generated in the rotary shaft due to the magnetization of the inner ring.
Meanwhile, if the inner ring is formed of a metal material having a weak magnetization property in order to avoid the problem that the heat is generated in the rotary shaft due to the magnetization of the inner ring, the hardness of the inner ring becomes lower so that the inner ring is weak against impact or the cost of the inner ring significantly becomes higher.